USA and Europe and other developed and developing countries face challenges in the areas of air pollution, public health, economic growth, energy security and national security as a result of overdependence on petroleum fuels. In January 2012, the Californian emissions trading scheme came into effect. This aims to reduce carbon dioxide emissions from the use of petroleum and other fossil fuels. In June 2012, the US Court of Appeals upheld the US Administration's set of clean car and fuel economy standards which aim to cut new car pollution, and petroleum use, in half by 2025.
A solution to the above problems is to develop a non-polluting, more secure and more sustainable transportation and energy economy utilising hydrogen. Indeed, this is recognised worldwide. Hydrogen is a high energy source with water as the non-polluting final combustion product.
At present, commercial hydrogen production relies mainly on the steam reformation of methane (natural gas). Over three quarters of the global production of hydrogen occurs using steam-methane reformation. In this process, steam and methane at high temperatures (about 1,000° C.) react to yield synthesis gas or syngas (a mixture of carbon monoxide and hydrogen). The carbon monoxide produced can be converted, by a subsequent water gas shift reaction, to carbon dioxide with the production of more hydrogen.
Commercial hydrogen production also occurs via the gasification of coal. In this process, steam and oxygen at high temperatures and pressures react with coal to yield syngas. Coal gasification is the oldest method of hydrogen production in both Europe and the USA.
Small commercial amounts of pure hydrogen are produced from the electrolysis of water. In this process, water is decomposed into hydrogen and oxygen using an electric current passed between two electrodes that are immersed in the water. Hydrogen is collected at the cathode and oxygen is collected at the anode.
The decomposition of water into hydrogen and oxygen by electrolysis at standard temperature and pressure is not favourable thermodynamically. Energy in the form of electricity or heat must be supplied. The reaction occurring at the anode can be represented by:Anode (oxidation) 2H2O→O2+4H++4e− E=−1.23V
The reaction occurring at the cathode can be represented by:Cathode (reduction) 4H++4e−→2H2 E=0.00V
Pure water conducts electricity poorly. If an appropriate electrolyte at an appropriate concentration is added to water, the electrical conductivity of water increases considerably. Care must be exercised in the choosing of electrolytes so that competition does not occur between the electrolyte and water to gain electrons at the cathode (reduction of cation) and to give up electrons at the anode (oxidation of anion).
Other methods of hydrogen production that are less common include biomass gasification, the carbon black and hydrogen process, photoelectrolysis, thermal decomposition of water, and photobiological production.
The production of hydrogen from methane produces large amounts of carbon oxides and produces several other pollutants and toxic by-products. Some impurities, such as carbon monoxide, are poisonous to humans and can be detrimental to various systems that require hydrogen—particularly hydrogen fuel cells containing proton exchange membranes. These impurities have delayed the utilisation of hydrogen fuel cells in automobiles and public transport.
The production of hydrogen from the electrolysis of water results in the least contaminated hydrogen product. Some pollutants may arise if electrolytes are added to the water to facilitate the process or to increase the velocity of the process, or if other substances are present in the water. Pollutants may arise particularly at the anode with the oxidation of anions (anode mud, etc.). Some pollutants may occur at the cathode from reactions with protons and electrons and substances present in water (carbon compounds for example). Either damage to, or dissolution of, electrodes may occur and the replacement of electrodes results in substantial financial costs. However, in principle, the production of hydrogen by the electrolysis of water should minimise considerably the overall production of carbon dioxide, pollutants and toxic by-products compared to other methods of hydrogen production.
Hydrogen can be used as a fuel directly in an internal combustion engine. Some automobile companies produce automobiles that can combust either hydrogen or gasoline. Because of its relative purity, the hydrogen produced by the electrolysis of water can be utilised also in hydrogen fuel cells. In a hydrogen fuel cell, as with hydrogen combustion, water is the final product. Vehicles in cities that operate utilising either hydrogen fuel cells or hydrogen combustion produce negligible pollutants compared with vehicles combusting gasoline or methane or other fossil fuels. The large scale use of hydrogen, produced by electrolysis, either in fuel cells or in internal combustion engines of vehicles would diminish city air pollution very significantly.
In addition, those countries that import oil and petroleum fuels can utilise hydrogen as a general energy source and become economically less dependent on oil and petroleum fuel imports. Amongst a range of other advantages, a hydrogen economy is an economy that has energy security, and hence, national security. Hydrogen is not only the cleanest energy available but it has the highest energy content of all fuels on a weight basis. The energy content of hydrogen is about three times higher than gasoline, natural gas, and propane on a weight basis.
Hydrogen also is an essential component in the production of ammonia and a range of other compounds. The most important use of ammonia is as an agricultural fertiliser. Its importance arises also from its conversion into a wide range of nitrogen containing compounds. A source of uncontaminated hydrogen and ammonia is vital for a clean chemical and food industry.
At present, the cost of producing hydrogen from the electrolysis of water is many times the cost of producing hydrogen from methane. This high cost occurs because electrolysis in practice does not meet efficiencies that are possible in theory. Overpotentials are needed to overcome interactions at the electrode surface. Competing side reactions at the electrodes result in various products and pollutants and less than ideal Faradaic efficiency. In addition, much energy is lost as heat because of the difficulty in finding suitable electrodes—particularly anodes. The cost of hydrogen production from electrolysis is a linear function of the cost of electricity.
In the Sabatier reaction, carbon dioxide is converted to methane in the presence of hydrogen. For the Sabatier reaction to be economically viable, large amounts of hydrogen need to be produced at relatively low cost. The reaction has been studied extensively as a means of converting carbon dioxide emissions, from fossil fuel combustion, to methane. The methane produced is then capable of further combustion. NASA intends using the Sabatier reaction on the space station to produce water for consumption by astronauts and as a means of utilising atmospheric carbon dioxide on Mars to produce methane for fuel. Carbon dioxide recycling from power plants and other industries via the Sabatier reaction is recognised as a major means of capturing and utilising carbon dioxide. In this reaction, carbon dioxide and hydrogen react in the gaseous phase, which avoids expensive carbon dioxide capture, transport and geologic sequestration. The Sabatier reaction can be represented by:CO2+4H2→CH4+2H2O
There is a need to decrease the cost of hydrogen production from the electrolysis of water. There is a need to produce hydrogen from the electrolysis of water without the production of pollutants or toxic by-products. There is a need to identify electrolytes or catalysts to facilitate the electrolytic process or to increase the rate of the electrolytic process, preferably without producing side reactions at the electrodes or pollutants or toxic by-products and without causing damage to electrodes. There is a need to decrease the power utilised in the electrolysis of water for hydrogen production. There is a need to decrease the reaction overpotential for the four electron oxidation of water to oxygen at the anode. There is a need to identify chemical catalysts and/or electro catalysts that can be utilised in the electrolysis of water to maximise the production of hydrogen per unit of electricity.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages. It is a further object to at least partially satisfy at least one of the above needs.